People that write standards and work on committees that write standards are working on a thing.
SMPTE have chosen github because it has the other stuff to allow them to manage the committee work, handle the issues raised in committees, drafts, tagging different versions, dealing with the committee processes etc.
They could have chosen something like JIRA, so at least we've avoided that.
That's what they said in the post about it being open.
They've moved the internal email mailing lists and other workflow to github as well as using git for the version control of the source.
Right. But people that write standards and work on committees that write standards are going to use whatever it is that the coordinating org wants, or whatever they can agree on, and they by definition will have access to it. For everyone else, none of that is of any relevance (at least beyond the ability to listen in on the process as it happens).